I am writting a story, which will be posted on here when it's done, that features a WAM gameshow now the losers punishment will be to be messed (in a pillory) whilest wearing a humiliating outfit. But what should the outfit be?

That will depend very much on who the loser is. I'm assuming a woman as the thread is tagged female.

A woman who only usually ever wears very smart skirt suits, dresses, and high heels, might find it humiliating to have to wear a boilersuit and wellies.

Equally a woman who's usually very practical and lives in overalls and boots might find it humiliating to have to be seen in a bikini, or a short skirt and skimpy top.

Someone who usually looks really smart might hate having to wear a baggy grey jogging suit.

Most (but not all) people would find having to do it naked humiliating.

Some people wouild find it humiliating to be dressed up in their old school uniform, or school sports uniform - while many guys loved the look, having to wear gym knickers put many girls off sports for life.

I think you need to give us more info about the character of the loser of the game, what they like and dislike, etc.

The issue with this is that the game show would have to chose the outfit before knowing who the loser would be. The six women competing have quite different characters. I'm trying to think of a universally humiliating costume.

The trouble with that will be that anything universally humiliating - perhaps a very baggy, badly-fitting clown outfit - is also going to be very unsexy to all but a tiny niche of wammers, and hence kind of defeat the point.

I'd suggest picking an outfit you want to see the looser messed up in, and then write into the story that before filming, but when the contestants were known, an AI was tasked with picking an outfit they would all hate, and perhaps describe the scene when it's revealed and their different, but universally "arrrghhhh!" response, knowing that one of them is going to end up wearing it.

Universally humiliating you ask? Something like a piggie outfit (pig nose, ears and tail butt plug.

Also Google Hucow for the cow based equivalent,,,

But generally also any cheap slutty outfit is humiliating for most girls

Some of the sissy/adult baby outfits can be pretty humiliating.
[even if its your 'thing' you probably won't want to be wearing it on a gameshow]
Something like https://www.ebay.co.uk/itm/145044057037

Making them wear underwear on the outside is also a great forfeit for losing a challenge mid way through the show.
Or just random extra clothing in general- especially if it colour/theme clashes or it's worn on the wrong body part; knickers/bra work on the head, baby bonnet/baby bib, big granny bloomers, mis-matched shoes/boots, adult diaper (on the outside), pink fluffy tutu etc...

Inflatable armbands and/or a swimming ring also has the great one-liner of telling the contestant to wear it because s/he's out of his/her depth.

Probably going to go with a French Maid outfit as that would seem to fit with both humiliating but also sexy.

I like some of these other suggestions and might work them in to other rounds of the 'show'